This invention relates to short reach optical interconnects consisting of optical emitters in the form of VCSEL (Vertical Cavity Surface Emitting Laser) arrays and photo detector array for transmitting data through optical fibers, and in particular to techniques for enabling multiple optical emitters to transmit over a single fiber.
Data communications rely heavily on fiber optic transmission to achieve the required speed and bandwidth. Electro optical transducer arrays for converting electrical data signals into optical emissions are coupled with careful alignment to fiber bundles for transmitting the light signals over both short and long distances to photo detector arrays which convert the light back into electrical signals for further processing.
The current implementations of these fiber optic interconnects commonly couple a matrix array of N VCSELs configured in a row/column pattern at an industry standard pitch, emitting light signals at an industry standard frequency, to an equal number of single or multi mode optical fibers of industry standard diameter. A single VCSEL is allocated a specific surface area footprint nominally equal to the row/column pitch, and transmits its light pulses from its vertical cavity, which is uniformly disposed in a common position within the footprint. The light is directed into a first end of a single optical fiber and hence to a single detector of the same frequency at the other end of the fiber. This makes up a channel or optical link.
Conventionally, differential signal transmission implemented in the supporting circuitry switches the signal between two active transmitter/detector sets, so as to make signal detection easier and more reliable at the receiving end. The current practice requires two channels or VCSEL/fiber/detector links; where the high/low or 1 and 0 signal switching is alternated between the links for respective high and low signal elements. The received signals of the two links are recombined by supporting circuitry at the receiving end for processing.
Current industry practice is based on commercially available 850 nanometer VCSELs from a number of vendors, commercially available multi mode optical fiber and detectors that use quantum well structures with the same wave length characteristics as the VCSEL transmitters.
The technology for transmission of data by photonic means started with a single laser transmitted into a single fiber with a detector at the other end. These links were first developed for use by the phone companies and then later used for implementation of data transmission over the world wide web. The edge emitting 1300 nanometer lasers that are used to accomplish these links have grown from single frequency devices to multiple frequency devices that are used to accomplish wave division multiplexed WDM transmissions. The use of WDM provides the capability to transmit numerous data streams through a single multi mode fiber thereby reducing the number of fibers required to transmit massive amounts of data from different sources.
The implementation for short haul fiber optic data transmission which uses 850 nanometer VCSELs and detectors started with optical ether net which uses a single fiber per transmitter/detector pair per link. This has been expanded to implementations that use one-dimensional and two-dimensional arrays of these links to satisfy the industry bandwidth requirements. These implementations, however, still use a fiber per link. VCSELs and detectors that operate at 850 nanometers are used in these arrays in products currently on the market under the trade names of Parolli and TC48. (No claim is made to these marks.) The fiber and connectors used in these links are a significant cost driver for the products.
The current short haul fiber optic data interconnects are limited by two factors. The first factor is the speed limitations of the ASIC drivers and demodulators, which interface the electronic inputs and outputs to the photonic devices, which delineates the throughput bandwidth available. The second factor is, that to effectively meet the bit error rate requirements of the commercial industry desired 10xe2x88x9218 or more realistic 10xe2x88x9215, differential logic architectures have been implemented which currently require two fibers per interconnect. Both of these factors impact cost and bandwidth considerations. The instant invention is directed to these issues.
It is an object of the invention to provide for a reduction in the fiber count of equivalent commercial configurations by at least 2:1 with the potential for reductions in fiber utilization of 4:1, with the inherent attendant reduction in cost or the possibility of built in redundancy without significant added cost.
The invention, simply described, makes use of VCSEL arrays and detector arrays of commercially available pitch and operating characteristics that are compatible with commercially available optical fiber sizes. The array design is modified and fabricated as multi-frequency arrays with selected VCSELs and detectors arranged in a selected pattern operating at a selected frequency, and other VCSELs and detectors arranged in a related pattern and operating at another selected frequency. The modification to the standard commercial design, and fabrication of the modified array, is within the scope of the present fiber optic chip technology, and need only be attended with close attention and practice to achieve the required results.
The VCSEL arrays and corresponding detector arrays are arranged by the mask design and fabrication to provide groupings of VCSELs of different frequencies and corresponding detectors of the same frequency set, but still at the same average pitch as the original commercial design. Each transmitting group is then coupled to its respective receiving group by a single, multi-mode fiber of the same core diameter as contemplated by the original commercial array design, so that multiple frequencies, such as two frequencies for a differential signal transmission channel, can be transmitted through a single fiber. This technique requires only half as many, or fewer, fibers between the transmitting array and the receiving array, as compared to the equivalent commercial norm.